Write protected SD card

my SD card is driving me crazy, because it won't let me write files to it. Searching the net turns up zillions of the same silly tip: sliding the write protect tab on the side of the card. There are even Youtube videos that show how to do that!

Well, my camera has no problems writing to this card, but on my Mac it is always shown as protected. Very annoying when you try to put back original camera files.

Not that I understand, but it works now. What I did was writing something to the card on my Macbook Pro using the built in card reader. After I did that, I could also write to it on my desktop Mac with the USB card reader that didn't work before.